Magma Cryptosystems - The University of Sydney

Cryptanalysis
One important measure of a cryptographic text is the coincidence index. If we define
p i to be the probability of occurrence of the i-th character of the codomain alphabet in
a sample plaintext of ciphertext, then the coincidence index of the cryptosystem is the
value:
n∑
p 2 i ,
i=1
where n is the size of the alphabet. Recall that the sum of all probabilities is 1, therefore
the coincidence index is at most 1. The coincidence index is a measure of the probability
that two randomly chosen characters are the same.
To compute the coincidence index for a particular string of length N, we first compute
the number of occurrences n i of each character in the alphabet. Then the probability of
equality for two randomly chosen characters from that string is given by
∑ n
i=1 n i(n i − 1)
,
N(N − 1)
which we define to be the coincidence index of a string.
For random text (uniformly distributed characters) in an alphabet of size 26, the coincidence
index is approximately 0.0385. For English text, this value is 0.0661. Therefore
we should be able to pick out text which is a simple substitution or a transposition of
English text. Other languages will have an associated coincidence index, which
Another important tool for cryptanalysis Kasiski test. This is useful for determining
periodicity in ciphertext. Ciphertext of a Vigenère cipher has a period m such that for each
fixed j the mi +j-th characters are given simple substitution of the corresponding mi +jth
plaintext. If a frequently occurring pattern, such as THE is aligned at the same position
with respect to this period, then the same three characters will appear in the ciphertext,
at a distance which is an exact multiple of m. By looking for frequently occurring strings
in the ciphertext, and measuring the most frequent divisors of the displacements of these
strings, it is often possible to identify the period, hence to reduce to a simple substitution.
The Magma crypto package functions
Exercises
CoincidenceIndex, Decimation, and FrequencyDistribution
provide functionality for analysis of the ciphertexts in the exercises.
1. For each of the cryptographic texts from the course web page, compute the coincidence
index of the ciphertexts. Can you tell which come from simple substitution
or transposition ciphers? How could you distinguish the two?
2. For each of the cryptographic texts from the course web page, for various periods
extract the substrings of im + j-th characters. For those which are not simple
substitutions, can you identify a period?